Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 211751

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
151 PM EDT Sun May 21 2017

A stationary front will remain across the Carolinas today
before lifting north into the area as a warm front tonight. A
cold front will move through the area Monday. High pressure
will briefly return Monday night before low pressure develops
over the Carolinas Tuesday and moves northeast off the DelMarVa
into Wednesday. Upper-level low pressure will linger over the
area into Friday.


With high pressure over Quebec and a stationary boundary across
NC the Mid Atlantic remains in a damming situation. Low clouds
will remain plentiful through the day hindering insolation and
keeping temperatures near 70 F. It is also expected to remain
dry through the day...except for far SW portions of the CWA
where a late day shower could approach.

Front will return northward as a warm front overnight. This will act
to increase rain chances from SW to NE as WAA lift develops in
vicinity of frontal boundary. Prefrontal showers chances continue
overnight. Have increased PoPs to categorical overnight east of
the mountains. We are not expecting thunder with this as
atmosphere is quite stable.


Shower chances continue Monday morning as cold front approaches from
the west. Front will struggle to move through the area. By
afternoon...enough instability could reside ahead of the front
for a few thunderstorms to develop...though, airmass recovery
from morning rain showers casts doubt on how far west/north this
would occur. Best chance for thunderstorm development across
central VA into S MD. Weak shear/thermo space suggests
disorganized general thunderstorms would be the primary mode.

Respite from rain chances late Monday into Tuesday morning as high
pressure briefly builds into the area. Latest guidance develops low
pressure along frontal boundary to our south Tuesday and moves it up
to near the DelMarVa coast Tuesday night. This will act to bring
rain chances back to the area as we remain in the western quadrant
of the lows track. Tight gradient suggests western areas might not
see much rainfall...with best chances along/east of I-95.

Current QPF probs suggest 0.5/1/1.5 inches at the 25/50/75th
percentiles with the two systems through 8 AM Wednesday...with
highest totals along/east of I-95.


The main synoptic feature dictating the forecast in the extended
periods remains the deep trough axis pivoting across the Great Lakes
Tuesday and tilting negatively across New England by Friday. There
is model agreement that there will be a cutoff low somewhere in the
base of the trough Thursday; where and how deep understandably still
to be determined. Regardless, the period will be wet/unsettled.

Precipitation will be dictated by location of vorticity maxima and
enhanced by diurnal heating. PoPs will be likely or close to it.
However, there will be dry periods. Too much moisture within the
column to hope for much sun, although the upper layer does dry out
by Thursday. Regarding thunder, ECMWF suggests there will be mean
layer instability both Wednesday and Thursday PM, while GFS is
focusing on only Thursday. Don`t believe these details can be
resolved at this timeframe, so will preserve chance of thunderstorms
both days. No good reason right now to deviate temps much from model
blend...which is climo.

The trough axis will move northeast of the area Friday, providing
the area with northwest flow on the back side of the deepening
surface cyclone. That setup suggests that precipitation chances will
be decreasing as drier air infiltrates the area. Cold advection will
initially keep temperatures down. The warm-up begin Saturday with
the return of warm advection.


Ceilings remain in the low VFR area with bases 3.5-4.5 kft...
though brief periods of MVFR ceilings are possible. Warm front
lifts northward into the area late today and a cold front will
cross the area Monday. These features will allow rain to develop
with periods IFR (to perhaps locally LIFR) developing tonight
due to low ceilings. Improvement is expected later Monday as
cold front pushes east of the area.

Coastal low may create more aviation restrictions late Tuesday.

The forecast Wed-Thu will be unsettled, with numerous opportunities
for flight restrictions. However, showers (and perhaps
thunderstorms) will be hit-or-miss albeit numerous. The afternoon
and evening hours should be preferred for showers/storms, but it is
too soon to be detailed with forecast evolution.


Small Craft Advisory was issued to account for low-end SCA
criteria wind gusts starting late this afternoon through
tonight. Wind gusts may remain elevated Monday...though
confidence is currently low. Lighter winds expected late Monday
into Tuesday as high pressure briefly builds over the area. SCA
wind gusts could return late Tuesday depending on the track of
the coastal chances across southern portion of the
Bay...which is closer to the current forecasted track.

The waters will be under the influence of an upper level storm
system crossing the area Wednesday and Thursday. Numerous
showers/perhaps thunderstorms will result. Gradient flow likely
won`t be that strong, although a coastal storm could develop
late Thursday.


Easterly onshore has allowed tidal anomalies to rise rather quickly.
A Coastal Flood Advisory was issued for St. Marys County.
Anamolies increase further today...with minor tidal flooding
likely at Straits (Coastal Flood Advisory extends through
tonights cycle). Have issued a coastal flood advisory for
Annapolis for the overnight high tide cycle.

Evacuation from the estuary appears unlikely through at least Monday
(and perhaps longer)...which will keep tidal levels elevated with
continued minor flooding possible at the most sensitive sites near
times of high tide.


MD...Coastal Flood Advisory from 1 AM to 5 AM EDT Monday for MDZ014.
     Coastal Flood Advisory until 3 AM EDT Monday for MDZ017.
MARINE...Small Craft Advisory from 5 PM this afternoon to 5 AM EDT
     Monday for ANZ530>533-538>542.
     Small Craft Advisory from 8 PM this evening to 8 AM EDT Monday
     for ANZ534-537-543.
     Small Craft Advisory from 8 PM this evening to 5 AM EDT Monday
     for ANZ535-536.


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